The geometric and dosimetric effect of algorithm choice on propagated contours from CT to CBCTs
Nash, D. McWilliam, Alan Palmer, A. L Vasquez Osorio, Eliana
Nash, D.
McWilliam, Alan
Palmer, A. L
Vasquez Osorio, Eliana
Citations
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Abstract
Purpose or Objective
Efficient adaptive radiotherapy requires rapid plan
assessment based on the anatomy of the day to determine
whether plan adaptation is required. To determine the
relevant DVH parameters, contouring the organs at risk
(OARs) on the daily image is required. Manually contouring
in such a short timescale puts a significant burden on
oncologists. Using software to propagate the planning
contours onto the daily images can theoretically remove
this burden. Since there are many commercially available
options, the geometric and dosimetric differences of the
propagated contours should be assessed, before evaluating
their role in plan adaptation.
Material and Methods
Planning CT (pCT) contours and 5 shading-corrected CBCTs
[1] (for more accurate dose calculation) acquired weekly
throughout treatment for 10 randomly selected head and
neck cancer patients were used. The patients were replanned
with VMAT in RayStation v6.99 (RS). The spinal
cord (SC), brainstem (BS), parotids and larynx were
propagated using 2 strategies: from pCT to each CBCT
(pCT2CBCT) and in cascade, i.e. from the pCT to CBCT1 to
CBCT2. The pCT2CBCT strategy was used in ADMIRE,
Mirada and RS, and the cascade in ADMIRE and RS, for a
total of 5 methods. Mean distance to agreement (mDTA)
between propagated contours was computed between
pairs of contours for all propagating methods. BS and SC
were expanded to form PRVs. For the dosimetric impact,
the plan was calculated on each CBCT after setup
correction. The relevant DVH parameters were extracted
(D1cc for BS and SC PRVs, mean dose parotids and larynx)
for each contour and each method, and compared using
the Kolmogorov-Smirnoff test to determine statistically
significant differences. The doses were compared against
the clinical constraints for BS and SC PRVs (D1cc< 54 and
46 Gy respectively).
Results
All methods produced similar contours evidenced by the
range of the mDTA (up to 1.7 mm) (table 1). Figure 1 shows
the dose differences for the OARs. There were no
statistically significant differences except for SC PRV, with
no particular method outperforming the others. The
presence of significant differences on the SC is indicative
of the high dose gradients present in the area, confirming
that small variations on the propagated contours cause
larger dose changes. The median dose for both left and
right parotids showed a small increase for most methods.
The larynx dose remained similar with time. There were 6
instances of the spinal cord failing against clinical
constraint, and 5 for the brainstem - all were in cases of
high dose gradients near to the OAR. No particular method
failed consistently.
Conclusion
Most propagated contours were geometrically and
dosimetrically similar, regardless of method used. For the
SC, however, the variations were large enough to causes
significantly different D1cc. Clinician review of
propagated contours in areas of high dose gradients is
critical.
Description
Date
2020
Publisher
Collections
Keywords
Type
Meetings and Proceedings
Citation
Nash D, McWilliam A, Palmer AL, Vasquez Osorio E. PO-1727: The geometric and dosimetric effect of algorithm choice on propagated contours from CT to CBCTs. Radiotherapy and Oncology . 2020 Nov;152:S956–7.